This project will explore and characterize the mechanism of the age-related decrease in the activity of ecto-5'-nucleotidase (ecto-5'-NT) in T and B lymphocytes. Ecto-5'-NT will be purified from human liver or placenta and a monoclonal antibody will be developed to the enzyme. With the use of the antibody it will be determined if there is a shift in the number of T and B cells that have the enzyme with age or if there is a decrease in the specific activity of ecto-5'-NT per antigenic unit with age in T and B lymphocytes. Changes in the specific activity of the enzyme with age could be due to any of several post-translational modifications, e.g., changes in enzyme conformation, alterations in glycosylation or change in membrane structure that would affect the function of the enzyme and each of these mechanisms will be studied if appropriate. The potential effect of this age-related decrease in lymphocyte ecto-5'-NT activity will be evaluated at a biochemical level by measurement by high performance liquid chromatography of the intracellular concentration of nucleotides and deoxynucleotides in lymphocytes from young and old individuals. Lymphoblast lines established from subjects of varying age will be assayed to determine if the age-related reductions in ecto-5'-NT activity is maintained in tissue culture (fibroblast lines will also be evaluated). Such cells would provide an excellent model for further information about the normal aging process in humans, and specifically the aging of the immune system.